1. Field of the Invention
The present invention pertains to apparatus and methods for forming articles from particulate materials.
2. Description of the Background
Examples of the kind of apparatus and methods to which the present invention may be applied are given in U.S. Pat. No. 4,252,760 and U.S. Pat. No. 4,263,237.
As explained more fully in these prior patents, one current technique for forming articles of materials such as tetrafluoroethylenes begins with such materials in particulate form. The shapes of the particles are highly irregular. Thus, a batch of such particulate material may be placed in a mold and, more specifically, in a particulate chamber having opposed walls. One of the walls can be advanced and retracted toward and away from the other. For example, the advanceable wall may be the inner part of an elastomeric bladder. Fluid is introduced into the bladder under pressure to advance said inner part.
When the wall is advanced with sufficient force, the particles are compressed, and their irregular surfaces mechanically interlock with one another. The result is a body called a "preform," which while often highly brittle or frangible, is sufficiently self-supporting to retain its desired configuration temporarily, and with sufficiently careful handling. Such a preform is heated or sintered to set the plastic, resulting in a finished article which is neither porous nor brittle, but rather, extremely strong and substantially homogenous in texture. Similar techniques may also be employed to form articles from other types of particulate materials, e.g. metals, ceramics, etc.
Although the apparatus and methods disclosed in the aforementioned prior patents have been highly successful, there has been room for further improvement in the context of certain specific types of manufacturing projects.
For example, in the manufacturing of tubular goods, such as pipes, sleeves, fittings, etc., a manufacturer often has occasion to manufacture a number of different items which are more or less identical, except for diameters. For example, such a manufacturer may make pipes of various sizes and/or thicknesses. The mold in which the particulates are pressed to form the preforms is a relatively expensive item. It would be highly desirable to minimize the number of different molds which would have to be provided in order to make such items in many different sizes and/or thicknesses.
Another particular problem occurs where the item being formed is quite thick. In such instances, it is sometimes difficult to achieve a good quality article, with relatively uniform density, with a single pressing operation. In some cases in the past, it has been attempted to gradually build up a thick article by placing less particulates in the mold than required for the complete preform, pressing those particulates, adding more particulates, pressing again, etc. Thus, the preform would be built up in layers. The problem with this approach was that it resulted in seams or weakened areas at the junctures of the various layers, where the bonding was not sufficient to provide a good quality article.
Still another problem area is that of manufacturing long articles, specifically long tubular articles. Not only is a long mold expensive, bulky, etc., but once again, it is difficult to apply uniform pressure in such a mold, and consequently, difficult to achieve relatively uniform density in the finished product, particularly if the mold and product are tubular.